Isotopic exchange between deuterium and cycloalkane on palladium catalysts

Abstract

Results of isotopic exchange between deuterium and the hydrocarbons, bicyclo[3.3.1]nonane, cis- and trans-1,2-dimethylcyclopentane, on palladium-alumina catalysts at about 50°C are reported. An intermediate common to the exchange of both dimethylcyclopentanes gives nearly completely exchanged cis- and trans-1,2-dimethylcyclopentane in the ratio of one to about four. In addition to this two-set exchange process, there is a one-set exchange process unaccompanied by epimerization in which nearly all of the hydrogen atoms in one set exchange. Other processes lead to the exchange of a smaller number of hydrogen atoms.

The isotopic distribution patterns of exchanged bicyclononane are inconsistent with the occurrence of one-set exchange via alternation between monoadsorbed alkane and π-complexed olefin but support instead alternation between mono-adsorbed and di-adsorbed alkane. The mechanism of two-set exchange is considered in the light of these experiments. Several forms of the π-allyl mechanism and of the roll-over mechanism are consistent with observations.